Exhaust emission from coal fired furnaces or boilers such as those used in power plants, may contain a variety of different gases depending on the type of coal being burned. The amounts at which some of these gases could be emitted into the environment are regulated. For example, there are restrictions on the amount of nitrogen oxides which could be emitted into the atmosphere.
To reduce the amount of nitrogen oxides emitted into the environment, selective catalytic reactors have been added into the exhaust systems of coal fired furnaces and boilers such as at power plants. Ammonia is injected into a selective catalytic reactor (SCR) and mixes with the nitrogen oxides in the exhaust emission to produce pure nitrogen and water, thus reducing the amount of emitted nitrogen oxides.
Unfortunately, this retrofit of selective catalytic reactors to coal fired furnaces and boilers such as in power plants to reduce nitrogen oxides can also result in visible emissions known as “blue plume.” This begins with the oxidation of a relatively small portion of the sulfur dioxides in the exhaust emissions to sulfur trioxide. Sulfur trioxide and water vapor readily combine to form an acid mist. Small amounts of water vapor, which may or may not be sufficient to convert enough sulfur trioxide to acid mist to become visible, are available in the exhaust gases both from humidity in the combustion air and products of combustion of the small amount of hydrogen in the coal. If there is insufficient water in the exhaust gases, blue plume may form downstream of the plant stack after the sulfur trioxide combines with atmospheric moisture. Finally, if the plant has a wet SO2 scrubber, there is abundant moisture present to convert a high fraction of the sulfur trioxide to acid mist and a substantial portion of this mist escapes the scrubber. The source of moisture merely controls where the acid mist is formed and, thereby, how visible it will be.
In any case, sulfuric acid mist is formed which can come down to the surface of the earth as acid rain. The long term health effects of sulfur trioxide or blue plume are not well known. The amount of sulfur dioxide catalyzed to sulfur trioxide and thence to acid mist is a relatively small fraction of the total sulfur from the coal burned. Nevertheless, the problem of blue plume is of great concern as evidenced by the purchase of most of the town of Chesire, Ohio by American Electric Power Co. (AEP) because of blue plume emitted from AEP's power plant which was upwind of the town of Chesire.
Several methods for essentially eliminating the acid mist and associated blue plume resulting from sulfur trioxide have been proposed. At least one has been reported successful. This method involves injecting ammonia (at much higher dosages than are used in the SCR) to the exhaust gas at a point where its temperature has been reduced to the range of about 250–300° F. (although the injection is sometimes carried out at higher temperatures). At this temperature, the ammonia reacts readily with the sulfur trioxide to form a number of possible compounds, the most notable of which are ammonium sulfate and ammonium bisulfate. The formation of these compounds by this reaction can capture a very high fraction of the sulfur trioxide thereby eliminating the conversion to acid mist and formation of blue plume.
This method of controlling blue plume, while highly effective, has several disadvantages. First, the large usage of ammonia constitutes a substantial expense. Second, if insufficient ammonia is injected, the reaction produces mostly ammonium bisulfate which is a very sticky, somewhat liquid substance at these temperatures. Ammonium bisulfate can quickly build up on downstream equipment and cause operational problems. Finally, even when sufficient ammonia is used to assure predominately ammonium sulfate (a dry powder) is formed, the ammoniated compounds are collected with and detrimental to the quality of the fly ash. The most advantageous use for fly ash (highest value and largest quantity) is in concrete as a partial replacement for Portland cement. Even the smaller quantities of ammoniated compounds that sometimes results from SCR use are problematic for this use of fly ash. The much larger amounts of these compounds from blue plume control by ammonia injection make the ash completely unusable. Unusable ash is typically disposed of in landfills. However, since many of the ammoniated compounds (such as ammonium sulfate) are water soluble, it is not clear that landfill disposal will be acceptable for ash containing large quantities of these compounds.